Transmission control network



A. ALFORD 2,495,174 TRANSMI S S ION CONTROL NETWORK Filed May 27, 1942 YlNVENTOR' Patented Aug. 6, 1946 TRANSMISSIGN CONTROL NETWORK Andrew Alford, New York, N. Y., assig'nor to Mackay Radio and Telegraph Company, New York, N. Y., a corporation of Delaware Application May 27, 1942, Serial No. 444,660

4 Claims. 1

This invention relates to high frequency transmission systems and more particularly to impedance matching networks for high frequency lines.

It is often necessary to provide high frequency lines with networks or the like in order that the line may be matched in impedance with a load. A number of such systems have been proposed such as short sections of transmission line, commonly called building-out sections, or other networks coupled to the line. In many cases it is found that the known forms of networks may be undesirable because of the difficulty of applying them to installations already existing or because of their size or position.

It is an object of my invention to provide an impedance adjusting network which is readily applied to any transmission line and which is relatively small and light so that it will not add materially to the weight or wind resistance of a line when applied thereto.

According to a feature of my invention the stated objects may be achieved by coupling to the transmission line a conductor of given characteristic impedance value, the length being determined by the characteristic impedance of the line and the standing wave ratio in the line and the position along the line being determined by the selected length and the characteristic impedance of the line.

A better understanding of my invention and the objects and features thereby will be had by the particular description thereof made with reference to the accompanying drawing in which Figs. 1 and 1A are diagrams used to explain the principles of my invention;

Figs. 2 and 3 are curves showing characteristics of networks used in myjnvention;

Fig. 4 illustrates one form of my invention applied to a line feeding an antenna.

Fig. 5 illustrates a further modification of my invention; and

Fig, 6 illustrates still another form of my invention.

The conventional forms of impedance matching networks are often cumbersome, particularly when used on high frequency supply leads to radio antenna. I have discovered that an impedance matching may be achieved by use of a simpler network consisting of a relatively long narrow conductive arrangement fastened to the conductors,

In Fig. 1 the effect of the network is demonstrated. In this arrangement a high frequency source I is shown connected over a transmission line 2, to a load impedance 3 which is made substantially equal to thecharacteristic impedance of line 2, In such a transmission line there will be no standing waves. If, then, a conductor 4 is coupled to the line it is found that standing waves are produced on the line, as shown at 5. The conductor has a characteristic impedance of Z0 and a length 1. With such a relationship it is found that the spacing of the conductor from the standing waves maximum, Imax is given by the value P. Since such a network is capable of producing standing waves in a matched line, it is clear that a similar network applied in a transmission line normally having standing waves of the same Imax to Imin ratio as the waves 5, and spaced the same distance P from the Imax position will serve to match the frequency of the line to the load in accordance with the reciprocity theorem.

In Fig. 1A is shown such a system in which the source I is connected over a transmission line 2 to a load 3a, the line normally having standing waves as indicated at 5a. The network 4 of length Z and characteristic impedance Z0 serves to match the impedance f the line as illustrated in this figure. t can be seen that this form of network will readily serve to secure impedance match in most transmission lines if the proper dimensions and position are chosen. Furthermore, since the network is of small vertical dimension it will exert very little bending stress on the line during high winds or the like, and is, therefore, particularly Well suited for use on open wire high frequency transmission lines which are exposed to the weather.

In order properly to select the desired network of dimension to satisfy the impedance matching requirement, it is possible to provide curves to aid in its selection. Such curves are illustrated in Figs. 2 and 3.

In Fig. 2 is shown a family of curves, representing different values of the ratio of characteristic impedance Z0 of the line to the characteristic Z0 of these networks, plotted with the length of the units in electrical degrees against the standing wave ratio Q, that is Fig. 3 then shows a set of curves for determining the spacing P, the spacing 'P being shown in electrical degrees plotted against the length Z in electrical degrees.

When it is desired to apply a network to match a given standing wave ratio in a line, the curves of Fig. 2 are first consulted. Assume, for example, the standing wave ratio is 3. It is seen that there are three sets of networks which will be satisfactory depending upon the characteristic impedance of the networks which it is desired to use. The characteristic impedance of the network may be readily calculated for any fiat metal objects of the type illustrated in Fig. 1 or for metal rod forms such as shown in Fig. 5. Accordingly, the desired width of plate or diameter of rod is found by calculation and the length is then determined by reference to Fig. 2. We will assume first that curve 22 is chosen. It will be seen that this requires a network having a length of substantially 60 electrical degrees. Turning then to Fig. 3, the length Z found from Fig. 2 is laid out on the horizontal axis, and the spacing P can'then be determined from this. In the given example it is found that the spacing P must then be made of the order of 2.5 degrees from the current maximum.

It is clear that any desired, impedance matching network may be secured from a family of curves as shown in Figs. 2 and 3. While only a few curves have been shown it is readily apparent that the number of curves which may be provided is substantially infinite, and, therefore, the variety of networks that can be provided in accordance with my invention is also infinite.

One advantage of the fiat sheet-like network is that it may be readily applied and adjusted experimentally without reference to the curves. In Fig. 4. is illustrated an arrangement in which transmission line 48 is coupled to a dipole antenna 4|. Matching sections 42, 43 are provided in each of the lines. If these sections are found to be too short, additional sections 44, 45 may be hung on the ends thereof. Further, if the sections 42, 43 are found to be too long they y be readily trimmed oif, being made of light sheet metal.

In Fig. 5 is illustrated how another form of network comprising sections of line 52, 55 of different diameter than the conductors of line 3! may be applied to the line. Th impedance of the line sections may be readily calculated and they may be applied to the line by use of the diagrams of Figs. '2 and 3 or similar diagrams calculated for this form of conductor.

While the solid metal arrangements shown in Fig. 4 may be provided to secure the desired impedance matching, such elements may be undesirable on outside lines due to the wind resistance. It is therefore clear that in place of solid metal sheets as shown therein wire screening or netting may be used.

Although sheets of screening or thin metal may be provided for achieving the impedance matching as described above, it is often desirable to build up units in the field out of wire loops. In Fig. 6 is shown a form of network applied to a transmission line 46 connected to an antenna 4!, in which each of the units BI, 52 comprises a plurality of small loops of wire fastened to the transmitting conductor. If desired these wire loops may be reinforced by a third wire 63, 64, as shown. Such a construction is preferable to the netting or screening since the openings are much larger and do not facilitate the formation of sheets of sleet. The impedance matching with such networks as shown in 63, 64 can be accomplished in a similar manner to that described above. In addition, however, the network such as shown at El, 62, has the additional advantage overthe use of the sheets in that the length of the units may be determined by the number of loops added to the line, so that the proper adjustment and length can be easily made in the field.

While the impedance of such networks as shown at 61, 62, may be calculated, this calculation is quite involved and it is often more feasible to find the impedance thereof by experiment. The impedance of such networks may be determined by use of the arrangement similar to that shown in Fig. 1. Since the standing wave ratio on the line which is created by the unit networks can be readily measured, and the characteristic impedance Z0 is known, the characteristic impedance of any unit of length I may be readily calculated after taking the measurements from the known functions. In order to secure the required data loops of a particular size may be strung on a matched impedance line such as shown in Fig. 1, and the information as to the standing wave ratio produced thereby determined for different lengths of network having a given looping dimension. Thus, a family of curves similar to those shown in Fig. 2 may be readily plotted experimentally. Likewise, the spacing P may be determined at the same time and a set of experimental curves similar in type to that shown in Fig. 3 may be plotted. When it is desired to match the impedance of a transmission line, the standing wave ratio of the line may be first measured and the desired length of loop network may be derived from the sets of curves in a similar manner to that previously described. When this has once been determined all that is necessary is to fasten the conductor forming the loops to the transmission line at one point and proceed to make the number of intermediate loops necessary to produce the desired length of network. It is, therefore, seen that this particular form of network arrangement shown in Fig. 6 is readily applicable to transmission lines in the field and may be applied by any attendant who is in charge of the transmission lines.

While I have disclosed specific apparatus for accomplishing this result in accordance with my invention, it should be distinctly understood that these are given merely by way of example, and not as limitations. In fact, any desired form of impedance network can be used so long as the conductor is chosen with respect to the unit impedance value and taking into consideration the characteristic impedance of the line, the standing wave ratio of the line and the proper position determined by this characteristic impedance and the selected length.

What is claimed is:

1. In a high frequency transmission the combination comprising a high frequency transmitting line complete in itself for transmitting energy to a load, a separate readily applied impedance matching network comprising conductive means having a given characteristic impedance connected to a conductor of said line throughout substantially the entire length of said conductive means, said conductive means having an overall length determined from the selection of a characteristic among those existing between standing wave ratio in said transmission line and network lengths, for different ratios between characteristic impedance of said line and of networks including approximately said given characteristic impedance, and said conductive means having a position along said line with respect to the standing wave maximum of standing waves in said line determined by the selection of one among the existing characteristics between network lengths and positions for difierent ratios between the characteristic impedance of said line and of networks including approximately said given characteristic impedance.

2. An impedance matching network according to claim 1 wherein said conductive means comprises a metal sheet fastened to said conductor.

3. In a high frequency transmission the combination comprising a two-conductor high frequency transmitting line complete in itself for transmitting energy to a load, a separate readily applied impedance matching network comprising conductive means connected to each conductor of the line throughout substantially the entire length of said conductive means, said conductive means defining a predetermined area in the plane of said conductor and having a given characteristic impedance and an overall length determined from the selection of a characteristic among those existing between standing wave ratio in said transmitting line and network lengths for different ratios between characteristic impedanc of said line and of networks including approximately said given characteristic impedance, and said conductive means having a position along said line with respect to the standing wave maximum of standing waves in said line determined by the selection of one among the existing characteristics between network lengths and positions for different ratios between the characteristic impedance of said line and of networks including approximately said given characteristic impedance.

4. A high frequency transmission line system comprising a high frequency transmitting line including an impedance matching network comprising conductive mea-ns having a given characteristic impedance connected to a conductor of said line, said conductive means having an overall length determined from the selection of a characteristic among those existing between standing wave ratio in said transmission line and network lengths, for difierent ratios between characteristic impedance of said line and of networks including approximately said given characteristic impedance, and said conductive means having a position along said line with respect to the standing wave maximum of standing Waves in said line determined by the selection of one among the existing characteristics between network lengths and positions for different ratios between the characteristic impedance of said line and of networks including approximately said given characteristic impedance, said conductive means comprising a series of looped wires each terminating at said conductor.

ANDREW ALFORD. 

